1. Field of the Invention
The present invention relates to a modem apparatus, communication control apparatus and communication terminal apparatus with the function of performing communications based on communication procedures conforming to ITU-T Recommendations, and the communication control method therefor.
2. Description of the Related Art
Conventionally, a communication terminal apparatus provided with a modem for performing communications based on communication procedures conforming to ITU-T Recommendation V.34 (hereinafter, referred to as Recommendation V.34) initiates a communication according to a sequence specified in ITU-T Recommendation V.8 (hereinafter, referred to as Recommendation V.8). Specifically, in response to a call placed from a calling terminal, an answering terminal transmits an ANSam signal to the calling terminal. When the calling terminal detects the ANSam signal, the calling terminal transmits a CM signal to the answering terminal. When the answering terminal detects the CM signal, the answering terminal transmits a JM signal to the calling terminal. Thereafter, the communication procedure shifts from that conforming to Recommendation V.8 to that conforming to Recommendation V.34.
In the case where transmission and reception of procedure signals are performed according to the sequence specified in Recommendation V.8, when the calling terminal incorrectly recognizes the procedure signal transmitted from the answering terminal, the calling terminal and answering terminal each continues transmitting the procedure signal based on the different communication procedure until receiving an expected signal. The case sometimes occurs thereby where communication operations are looped in the calling terminal and answering terminal.
Japanese Unexamined Patent Publication HEI-9-116718discloses a communication terminal apparatus provided with a modem for performing communications based on the communication procedure conforming to Recommendation V.8. Despite the expected procedure signal being the JM signal, the communication terminal apparatus detects a DIS signal based on the communication procedure conforming to Recommendation T.30 that is transmitted incorrectly from the answering terminal, after transmitting the CM signal. Specifically, the communication terminal apparatus judges a flag pattern of the DIS signal to detect the DIS signal.
However, in the above-mentioned communication terminal apparatus, since the DIS signal is detected by judging the flag pattern of the DIS signal, procedure signals having no flag pattern cannot be detected.
Examples of signals having no flag pattern are an AC signal that is transmitted from an answering terminal to a calling terminal based on the communication procedure conforming to Recommendation V.32 or V.32bis as illustrated in FIG. 1 and a USB1 signal that is transmitted from an answering terminal to a calling terminal based on the communication procedure conforming to Recommendation V.22 or V.22bis. Accordingly, the above-mentioned communication terminal apparatus cannot detect such procedure signals. Therefore, in the case where such procedure signals are transmitted from the answering terminal to the calling terminal, the calling terminal continues transmitting the CM signal based on the communication procedure conforming to Recommendation V.8. On the other hand, the answering terminal continues transmitting the USB1 signal based on the communication procedure conforming to Recommendation V.22 or an AC signal based on the communication procedure conforming to Recommendation V.32. As a result, there is the problem that communication operations are looped in the calling terminal and the answering terminal. For example, the above-mentioned AC signal and USB1 signal are signals used in data communications. Since the above-mentioned communication terminal apparatus cannot detect such signals, there is the problem that a data communication cannot be performed in such apparatus.
An object of the present invention is to provide a modem apparatus, communication control apparatus and communication control method capable of responding to communication procedures specified in an arbitrary Recommendation including a communication procedure conforming to Recommendation V.34, a facsimile communication procedure conforming to Recommendation T.30 and a data communication procedure based on, for example, Recommendation V.22, and of assuredly preventing communication operations in the apparatuses from being looped.
In the present invention, a calling terminal receives a procedure signal conforming to a predetermined Recommendation from an answering terminal, and detects signal components each at one of a plurality of frequencies in parallel from the procedure signal. Based on the detected result of the detected signal components each at one of a plurality of frequencies, the calling terminal outputs an identification signal indicative of a type of Recommendation to a host, and controls the switching between operation modes according to the instruction from the host based on the identification signal.